1. Field of the Invention
The present invention relates to an optical receptacle to be used for optical communication through an optical fiber, and to an optical receptacle module including the optical receptacle and an optical element.
2. Related Art
The recent popularization of wide area network such as the Internet and Intranet, and the sharp increase in communication traffic have been promoting the expansion of optical communication network. In particular, the technique of high-speed optical communication through an optical fiber (hereinafter, “optical fiber communication technique”) is rapidly spreading in the market, as this is employed for attaining a large transmission capacity in the field of data storage, in addition to the use in the Fiber To The Home (FTTH) system, in which the optical fiber is drawn into each home. In an optical communication apparatus an optical transceiver is incorporated, and serves to convert an optical signal to an electrical signal, and vice versa.
The optical transceiver generally includes therein an optical receptacle module removably connectible to an optical connector plug. The optical receptacle module is constituted of either or both of a combination of a light emitting element and an optical receptacle, and a combination of a photodetector and the optical receptacle. The optical transceiver is designed in compliance with standards such as Small Form factor Pluggable (SFP), 10(X) Gigabit EtherNet transceiver PAcKage (XENPAK), and 10(X) Gigabit Small Form Factor Pluggable (XFP). Conventional techniques related to the optical receptacle module employed in such optical transceiver can be found, for example, in JP-A No. 2004-258164, JP-A No. 2004-325605, JP-A No. 2003-107288, JP-A No. 2007-079422, and JP-A No. 2001-66468.
FIG. 1 schematically shows a cross-sectional structure of the optical receptacle module (optical module) 32 according to JP-A No. 2004-258164. The optical receptacle module 32 receives a frontal facet of a plug ferrule 33a of an optical connector 34, which is butted to a frontal facet of a ferrule 21a constituting a part of a fiber stub 23, thus to be optically coupled. The through hole of the ferrule 21a includes an optical fiber 21b, and the through hole of the mating plug ferrule 33a also includes an optical fiber 33b. At a rear end of the optical receptacle 28, an optical element 29 is provided. The optical element 29 is either a light emitting element or a photodetector, fixed in a housing 31 to be optically coupled with the optical receptacle 28 via a lens 30. The optical receptacle 28 includes a fiber stub 23, a sleeve 25, a housing 26 and a holder 27. A frontal facet 23a and a rear facet 23b of the fiber stub 23 are both mirror-finished, to minimize connection loss of an optical signal.
The sleeve 25 is enclosed in the housing 26 fixed to the holder 27, and serves to fasten the outer circumferential surface of a tip portion of the fiber stub 23 to thereby stabilize the optical axis of the fiber stub 23. Once the frontal facet of the plug ferrule 33a is inserted into the housing 26 and butted to the frontal facet of the ferrule 21a, the optical fiber 33b in the plug ferrule 33a and the optical fiber 21b in the ferrule 21a are optically coupled, thereby enabling exchanging optical signals between the optical fibers 21b and 33b. 
The present inventor has discovered, however, that the structure of the conventional optical receptacle does not always secure sufficient load resistance, for the following reason. The optical receptacle module is connected to various equipments through a patch cord that includes a plug ferrule. In some occasions a plurality of patch cords is tied in a bundle, which is a situation where each plug ferrule is prone to suffer a lateral load or a momentum load. In case such load is imposed in excess of the elastic retention force of the sleeve enclosed in the optical receptacle, positional displacement, in other words optical axis displacement takes place between the frontal facets of the ferrules, which generates connection loss of the optical signal. Such connection loss may jeopardize the power budget in the optical communication system. Also, the performance expected from the optical transceiver largely depends on the performance of the optical receptacle module to be incorporated therein. Accordingly, the optical transceiver is required, as a part of the performance thereof, to suppress the fluctuation of the connection loss of the optical signal originating from the load imposed on the optical receptacle to be, for example, 1.5 dB or lower in the case of an SC-type optical receptacle, and 0.5 dB or lower in the case of an LC-type optical receptacle. Such performance will hereinafter be referred to as “load resistance”.
JP-A No. 2004-325605 discloses a reinforced structure in which one of the open ends of the sleeve in the longitudinal direction is made thicker than the other open end. In such reinforced structure, however, since the former open end has greater retention force than the latter open end, when the lateral load or momentum load is imposed on the optical receptacle, the former open end may act as a lever fulcrum with respect to the ferrule inserted into the sleeve, thereby increasing the optical axis displacement, and hence the connection loss. Besides, partially increasing the thickness of the sleeve leads to an increase in manufacturing cost.
The present invention has been accomplished in view of the foregoing situation, and provides an optical receptacle with a sleeve of a relatively simple structure, yet capable of providing sufficient load resistance, and an optical receptacle module including such optical receptacle.